Two thousandths of an inch

As I reported last weekend I put all my polymer coated lead bullets through a max case gauge and still had problems with my new STI. I had saved one of those cartridges to diagnose the problem.

The cartridge still failed to chamber when stripped off the magazine by the slide when I tried it again at home. The bullet in that cartridge was seated too deep by about 0.040”. None of the others were too deep so I suspect it happened during the chambering of the cartridge. I pulled the bullet and reseated it at the proper depth and it chambered just fine. Hmmm… maybe the crimp just isn’t tight enough, the bullet gets driven deeper into the case, then the cartridge fails to nose down into the chamber as it comes off the magazine.

I shot in another steel match yesterday to get more samples.* On the first stage the first few strings went fine then a round failed to chamber the entire depth. I couldn’t pull the slide back. The gun was essentially locked up. I dropped the magazine, held tight to the slide then pounded the grip forward with the web and palm of my hand. The extractor pulled the round out and it was ejected. I put in a fresh magazine and completed the string in 17.xx seconds. I switched to my Montana Gold JHP handloads for the rest of the match and had no more problems.

I brought home the problem round and put in the case gauge. It fit just fine. I put it in a magazine and tried to to chamber it. It failed to chamber all the way. I tried dropping it directly into the chamber and dropping the slide. It chambered but again I couldn’t extract it without slamming my hand into the grip.

But the round fits just fine in the case gauge!

I measured the round. At the largest point it is 0.425”. The specification for .40 S&W is 0.423. So, it is oversized by 0.002”. But CASE GAUGE!

Hmmm… Maybe I have another case gauge around here… I did. I had lost one for a while and purchased another. I pulled out the other case gauge and the cartridge failed, big time.

These pictures are of the same cartridge in two different case gauges:

IMG_3314IMG_3313

In the picture on the left I applied a couple ounces of pressure to get it to seat all the way. In the second picture I put the maximum amount of pressure I could comfortably apply with my thumb to get it in that far.

So why is just the ammo with the polymer coated lead bullets giving me problems?

I measured a few bullets. Depending upon which axis I measure the bullets they have a diameter of 0.400” to 0.403”. The specification is 0.401”. The Montana Gold JHPs I measured have a diameter of 0.399.

Here is the cartridge:

IMG_3315

You can’t really see it but you can feel a bulge where the base of the bullet is in the cartridge.

Five conclusions:

  1. With a slightly over spec bullet and probably max thickness brass** I end up with an oversized cartridge.
  2. The Midway case gauge, on the left, is slightly oversized.
  3. My gun has a minimum sized chamber.
  4. Two thousands of an inch can make a huge difference in the reliability of a gun.
  5. I must use the L.E. Wilson case gauge (on the right above) for this gun.

I might be able to use the Midway gauge for some other gun(s).

Scary thought… Can you imagine needing your gun in a life or death situation and losing the fight because of two thousands of an inch?


* I can’t use this ammo at indoor ranges and I don’t have easy access to any outdoor ranges except when I shoot at matches.

** Not all cartridges with polymer coated lead bullets fail the tighter case gauge. Only some of them fail.

18 thoughts on “Two thousandths of an inch

  1. Sure sounds like an encounter with “tolerance stacking.” How much would it affect your accuracy to lap out the chamber a tiny bit? Make sure it feeds anything within spec.

    • I agree with the tolerance stacking. But at this time I can’t say that the chamber doesn’t feed anything within spec. All the cartridges which have failed are out of spec, even if it is only 0.002″ out of spec.

  2. When I started shooting 10mm, I discovered that I needed to use the LEE resizing die to make my reloaded ammo fit the chamber.

    This was a pain in the rear, so I cleverly induced a local gunsmith to bore the chamber out another 0.02″.

    Which worked just swell for a couple of years, and then one day the sucker blew up because … duh! … the chamber was overbored!

    It cost me $700 (note, everything costs $700 nowdays) to replace the barrel, and the cracked STI grip, and a couple of other minor items.

    Moral of the story is: don’t take shortcuts. Full-length resizing, being very careful about your choice of bullets (both shape and seating depth) and using a ‘reliable’ chamber gauge may add a lot to your reloading process.

    But they will save you from the embarrassing (and expensive) KaBOOM!

  3. Another issue is that bullet fit & barrel diameter affects maximum pressure.

    In my case, I developed a near maximum load for a Hi-Power but when I tried it in a S&W Model 39, the case blew out down the feed ramp. On further investigation, I discovered the Hi-Power had a bore diameter of 0.359 while the Model 39 bore diameter was 0.354.

  4. You’re have the same sort of problem I had as soon as I started loading cast bullets. Those who shoot cast will often use a bullet that’s over-sized by one to three thousandths. That means, unless you’re customizing all of the cases you use with the cast bullets, your finished cartridge will be one to three thousandths over-sized. If you run it through a full length whuchamacallit after it’s assembled, then you’re probably sizing the bullet back down in the process, in which case you may as well have used a standard, jacketed-bullet-sized cast bullet.

    It sounds like your loopy-coat bullets aren’t sized at all. Either that or they’re being sized and then coated, and the coating is uneven. I’d switch brands, or if you’re really married to those bullets I’d size them for consistency and concentricity before loading them into cartridges.

    • I’m “married” to them in the sense I have a few thousand of them under my bench.

      I’ll think about it more and do some research. It could be if I use a certain brand of brass they will be fine.

  5. “I switched to my Montana Gold JHP handloads for the rest of the match and had zero problems.”

    Bummer. What was the nature of these zero problems? Was the gun hitting high, low, right or left? I’ve had zero problems too. Zero problems can be a bitch ; )

    (My company of course sells optic mounts. When someone says he installed one of our mounts and had “zero issues” or “zero problems” I have to do a little double take. Therefore I tend to avoid using the word “zero” when talking about firearm mechanics unless I mean to refer to sights and sight-in issues)

      • I was mainly just needlin’ ya. Playing with words, finding double meanings/possible misinterpretations, is a sort of hobby of mine. It pisses off some people, but one positive result is that both of my kids are better than average speakers and writers.

  6. I would suggest grabbing a handful of those coated bullets and doing some measurements. Can you spot remove the coating, or spin them and remove it in rings? This would be to find out how consistent and even it has been applied.

    Now that I think about it, the coating may be one of the factors in your set-back. Too slippery. Perhaps you could set up in a lathe (or simulate one) to remove the coating up to the point it would be seated. If thick enough, this could help solve your oversize issue. Or, would this not be considered kosher for the range?

  7. More people on castboolitis.gunloads.com are playing with powder coating their own cast bullets as an alternative to lube. So far the reports I’ve seen have been positive. One offshoot advantage is that different colors can be used to identify different bullets, different batches, different loads, or whatnot. I haven’t gotten into it, but as I recall at least some people are sizing the bullets after powder coating. That would seem to make sense as it should correct for variations in coating thickness.

    As-cast bullets are sometimes slightly out of round due to the nature and use of the bullet mold, and that is dealt with during sizing. It remains to be seen whether Joe’s are out of round or inconsistent in diameter to begin with or if the coating is uneven, or if there’s a combination of the two.

    Sizing of course adds another step to the loading process, and anyway it shouldn’t be necessary when using commercially produced bullets unless you want a custom size. The flip side is that having your own sizing setup allows you to play with different sizes, and matching the bullet size to the gun, according to the real serious cast bullet shooters, is vitally important– It’s another one of the variables, like the powder, charge weight, bullet weight, lube and bullet style, etc, that determines accuracy and reliability.

  8. Don’t put a round into the chamber on a 1911 and then drop the slide. It forces the extractor to stretch over the case rim. Eventually this causes the extractor to fail. The case is designed to slip up under the extractor as the round is stripped out of the magazine and the case head slides up the breech face.

  9. If you’re not already using one, maybe try using a Lee Factory Crimp die. Besides giving a good, solid crimp the FCD also has a sizing ring that can often fix issues like this. I don’t load .40, but I’ve been happy with the FCD for 9mm, .45 ACP, and several rifle cartridges.

    • I’d second the factory crimp die. I found that makes a much cleaner fit in a gauge when I do 45 acp.

      (Normally Rainer plated 230 grain or Lee 230 and 250 grain cast sized to .451″)

      Both for the good taper crimp, but also for the sizing ring.

  10. Pingback: Steel match results | The View From North Central Idaho

  11. This is scary in the sense that the initial gauge allowed you to think it wasn’t a sizing issue.

    A wise man told me….”Use new brass when it matters, in competition and self defense, and use reloaded brass for practice.”

    It’s what I do now, and so far no problems. It’s not a possibility for everyone, but it works for me.

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